Phototube



9 J. 1'. TYKOCINER ET AL 85,53

PHOTOTUBE Filed Jan. 5, 195B 2 Sheets-Sheet 1 w ,Uw"

Jan. 2, 1940. J. 1". "rYKoc|rER ET AL 2,185,531

' Filed Jan. 5, 1958 '2 Sheets-Sheet 2 aae g gi w Patented Jeni 2, 1940 I UNITED STATES PHOTOTUBE Joseph Tykoelnski Tykociner,

Jakob Rum, and

Lloyd Preston Garner, Urbana, Ill., asslgnors to the Board of Trustees of linois, Urbano, 111.

the University of 11- Application January 5, 1938, Serial No. 183,460

11 Claims. (Cl. 250-275) The object of our invention is to provide a phototube of greater sensitivity and stability than has heretofore been possible, with substantial uniformity of product. This is accomplished by subjecting the alkali metal forming the photosensitive surface of the cathode of such a tube to what we term bombardment with hydrogen, or other suitable gas, which has been highly activated and to a substantial extent dissociated, and with electrons. We have secured an increase of sensitivity by our method exceeding fifty times that previously obtainable.

In the drawings:

Fig. l is a schematic drawing'of one arrangement of apparatus which may be used in carrying out the several steps of the method of our invention.

Fig. 2 is a schematic drawing showing an activating and dissociating chamber equipped with filament instead of electrodes.

Fig. 3 is a schematic drawing showing an activating and dissociating chamber equipped with both electrodesand filament.

Fig. 4 is a schematic drawing of an arrangement of apparatus which may be used in carrying out the method of our invention automatically.

Fig. 5 is a schematic drawing showing an improved type of photo-tube constructed according to our invention.

. Fig. 6 is a schematic drawing of an arrangement of apparatus illustrating the use of the method of our invention in the formation of the photosensitive surface of an improved type of photoelectric photo-tube.

Fig. 7 is a schematic drawing of an improved type photoelectric photo-tube suitable for television known as an iconoscope.

Fig. 8 is a schematic drawing of an arrangement of apparatus for carrying out our improved method of forming the photosensitive surface of a photoelectric photo-tube.

Similar reference numerals in the figures indicate the same parts.

Referring to Fig. 1, in carrying out our invention with one arrangement ofv apparatus we employ a source of molecular hydrogen, container l, which may be a steel tank, equipped with a pressure reduction valve 2 and a supply valve 3. This container is charged with molecular or diatomic hydrogen'known. as H2, which is commercially available, or may be obtained from an electrolytic decomposing cell, or other suitable source. Valve 2 is adjustable so that hydrogen can be discharged at a suitable pressure, which may be of the order of 0.1 atmosphere to 1 atmosphere, depending on the pressure in other parts of the system.

' Suitably connected valves 2 and 3 is water trap 4 for control of the humidity or water content of the hydrogen. The temperature of water trap 4 is adjustable so that substantially all water, or as much as may be desired can be removed from the hydrogen. If desired, the water trap may be supplemented by a palladium thimble or other suitable means for obtaining substantially pure hydrogen or for obtaining hydrogen of definiti water content.

Connected with water trap 4, through pressure valve 5 preferably adapted to precise adjustment for controlling the fiow of gas, is activating and dissociating chamber 6, which may be of any suitable size or shape, depending on the desired capacity of the system. This chamber is equipped with activating and dissociating means, which may consist of a pair of electrodes 1, as shown in Fig. 1, at suitable potentials, a heated filament 8, as shown in Fig. 2, a combination of these, as shown in Fig. 3, or other electrical means for heating or ionizing. Or the chamber may be excited electrodelessly by high frequency currents. Or the chamber may be made of quartz or other suitable material and, a trace of mercury being preferably added. to the H2, subjected to ultra-violet light.

To chamber 6 is connected the phototube 9, which is to be sensitized. So that the system may be evacuated and the pressure therein further controlled, a side valve I0 is interposed between chamber 6 and tube 9, and connected with a vacuum pump.

The various connections may be of conventional form.

The photoelectric phototube may be of the type described in Letters Patent of the United States No. 1,381,474 to Jakob Kunz of June 14, 1921, or other conventional type. The form shown in Fig. 1 consists of a glass envelope or bulb ll, otherwise evacuated but containing a cathode 12 having a surface iii of alkali metal, for example, lithium, sodium, potassium, rubidium, or caesium; an anode ll of non-corrosive metal, for example, nickel, silver, or platinum; and a residuum of alkali metal l5 from which the cathode was formed. But we may employ a tube having multiple photosensitive electrodes or any form or arrangement of photosensitive alkali metal surfaces which lose electrons under the influence of radiation.

To sensitize the tube 9, the system having been with container I through V first evacuated through valve l0, and that valve closed, the valve 3 is opened and the valve 2 adjusted so that the contents of container I will be discharged into the system at the desired pressure, which normally does not exceed 0.1 atmosphere, the valve 5 being open and adjusted accordingly. As the hydrogen passes through water trap 4, its moisture content is reduced to approximately zero, or other desired extent. It then passes into chamber 6 and the remainder of the system, including the tube 9, until a pressure of about 0.5 mm. Hg is reached, when valve 5 is closed.

A velocity selector of known type may be interposed at or near the point of admission of the activated and dissociated gas to the tube as indicated at 20 in Fig. 1, for the purpose of regulating the movement of such gas into the tube.

A suitable potential is then applied to the electrodes 1. For example, using hydrogen having a humidity of 0.1%, at room temperature, the electrodes being spherical and of a diameter of 5 mm. and spaced 5 mm. apart, and at the pressure indicated, a potential of about 750 volts applied for about five seconds usually produces a satisfactory result. We have secured our best results with the presence about the electrodes of atomic hydrogen indicated, during the application of potential to the electrodes, by a substantial predominance of the Balmer lines appearing in the spectroscope.

The sensitivity of the tube is then tested in the conventional manner and ordinarily will be found to have been substantially increased. If the desired sensitivity has not been attained, the system may be again evacuated through valve l and the operation repeated.

Sufilcient sensitivity of the tube having been attained in the manner described, we then subject the tube to additional treatment which further sensitizes and stabilizes it, in the following manner: An electronic gun l6 of the form used in cathode ray oscillograph tubes, including incandescent filament I 1 and anode cylinder l8, as shown diagrammatically in Fig. 1, or other suitable source of electronic discharge, equipped with electron optical elements for uniformly distributing electrons over the photosensitive surface, is connected to the tube as at l9. The tube is then preferably evacuated. Thermions emitted by the filament l1 are accelerated by the anode cylinder I8 and impinge upon the alkali surface l3 ofthe cathode l2. The rate of electronic discharge may be of the order of 0.01 to ma. per square centimeter of the area of the alkali surface of the cathode I2 for from one second to ten minutes, depending on the potential which may be from 20 to. 400 volts. The sensitivity of the tube is then tested at intervals of a few minutes and if the desired sensitivity has not been attained and retained, the operation is repeated. When an optimum result has been reached, the connections are removed, the tube evacuated, or filled with suitable gas, and sealed.

It will be understood that what we have referred to as cathode l2 serves as an anode during the period of electronic treatment.

It will also be understood that our method is equally applicable to the restoration of a phototube which has lost its sensitivity.

The sensitizing and stabilizing operations may be applied alternately, intermittently, simultaneously, or otherwise, the tube being tested for sensitivity and stability between times. This may be carried on until the optimum stabilized sensitivity is reached. When sensitizing and stabilizing operations are carried on simultaneously, it is desirable that the'gas pressure within the tube be so maintained as not unduly to impede the flow of electrons.

Our method may be carried out continuously with automatic control of the valve 5 by connecting it electro-mechani'cally with a suitable pressure indicating apparatus, such as an ionization pressure gauge, Pirani pressure gauge, thermic expansion gauge, or other similar apparatus, positioned within the system and so adjustable as to maintain the requisite pressure within the system. The essential element, that is, a heating wire whose resistance varies with change of pressure, of such a device, in the form of a Pirani pressure gauge, is shown at 32 in Fig. 4, the compensating element beingshown at 4|, the balancing resistances at 42 and 43, the relay at 45. A system of contacts 46, 41, 48, 49, 50, 5|, 52 and 53, which are actuated by a corresponding number of cams driven by a common shaft (not shown) is provided for opening and closing the several circuits and controlling the resistance. An auxiliary cam (not shown) is provided for opening and closing the valve III. In place of "cams, relays with rotating contacts may be used.

The operation of the system may be as follows:

For measuring the photosensitivity of a phototube, contact 46 is closed, contacts 41, 49, 49, 52 and 53 are open, contacts 50 and 5|, which are double contacts, are closed in lower position, valve I0 is open and valve 5 is closed. Filament 8, serving as an anode remains cold. A substantial vacuum is established in the tube 9.

For sensitizing the alkali metal surface l3 with activated and dissociated hydrogen, contacts 46, 50 and-5| are open, contacts 41, 48, 49 and 52 are closed, relay contact 53 and valve 5 are controlled by the pressure in the tube 9. Filament 8 is heated with full current, and the tube becomes filled with H2 at a pressure of about 0.2 mm. Hg which is activated and dissociated.

For further sensitizing and stabilizing th photosensitive surface I 3 thermionically, contacts 46, 48 and 49 are closed, contacts 41 and 52 being open, contacts 50 and 5| closed in upper position, relay contact 53 and valve l0 open, valve 5 closed. Filament 8 is heated with reduced current and a substantial vacuum is established in the tube 9.

In the case of continuous operation, the valves 5 and I 0 are timed in such a way that each opens and closes at suitable intervals. The application of potential to the electrodes 1, or of current to the filament 8, is made to vary correspondingly so that the potential will be highest and the flow of current greatest during high pressure periods when the gas is being activated and dissociated, diminishing during periods .of low pressure and evacuation when the electron bombardment takes place.

Alternating current may be applied to the photoelectric and electron emitting circuits. In such case, the use of alternating current whose two half waves difier in amplitude has been found to be advantageous, especially in enabling photocurrent measurement during the stabilizing operation. A direct current milliammeter 54 may be made to respond to the phase of large amplitude for measuring the thermionic emission, while a microammeter 55 will respond to the phase of small amplitude for measuring photoelectric emission or sensitivity.

photosensitive alkali metal surface with activated and dissociated hydrogen may be carried beyond a point of maximum sensitivity with resulting loss of sensitivity, and that in such case bombardment with electrons not only restores the lost sensitivity but produces greater sensitivity than previously had been attained.

We have indicated the use of substantially dry hydrogen and hydrogen of varying water content. When substantially all water vapor has been removed from the hydrogen prior to its introduction into chamber 6, we prefer to use a filament instead of electrodes in this chamber. In such case as shown in Fig. a tungsten filament 8 is preferably used'and heated by suitable electric current to a temperature of about 2600" K. for a period of about one to five minutes. In other respects the procedure is the same as when electrodes are employed in chamber 6.

Instead of molecular or diatomic hydrogen, water vapor may be supplied from a suitable source to the system through valve 5 and sub- ,iected to glow or other electrical discharges to produce substantial amounts of atomic hydrogen. When water vapor is used, it is introduced into the system at a pressure of the order of 0.01 to 0.1 mm. Hg. In such case we generally employ electrodes in chamber 6 and apply a suitable potential, which may be of the order of 1000 volts, for a period of about one to ten seconds. But a supplemental filament may be employed to reduce the breakdown voltage to advantage; or the incandescence of the filament alone may be relied upon to dissociate the water vapor. In other respects, the procedure is the same as in the first instance.

When water vapor is thus used, it may be desirable to dilute it with hydrogen, helium, argon, or other similar gas for the purpose of controlling the pressure and the breakdown potential of the mixture.

The sensitizing and stabilizing operations may be simplified by so constructing the phototube as to include certain essential elements, as shown in Fig. 5. In the support 2| are sealed four leads supporting the members l4, l2 and H. In the upper part of this support 2| a cavity is provided to receive a lump of alkali metal l5 introduced through tube 22 for sublimation to form alkali surface l3 of cathode l2. Anode l4 and cathode l2 with alkali surface l3 are of usual form and material. The filament I1 is electrically heated to activate and dissociate the molecular hydrogen or other gas which is introduced through the tube 23 and also to provide a source of electrons. Additional filaments may be provided for either purpose. In other respects the operations may be carried on substantially as when the arrangements of Fig. 1 and Fig. 3 are employed. Except when water vapor is used, anode l4 may be dispensed with as cathode l2 serves as an anodewhen the heated filament I1 is used as a source of electrons, and the filament I! will serve as an anode in the finished tube.

Instead of employing a structurally complete phototube, we may use a photoelectric phototube in which alkali metal has not yet been applied to the surface of the cathode, or if applied, insufficiently. In such case, we have obtained good results by first applying a thin layer of alkali metal to the proper surface of the cathode, then sensitizing, and then stabilizing and repeating the three steps until sufllcient sensitivity has been attained. The application of the alkali metal to the surface of the cathode and the sensitizing and stabilizing operations may, however, be carried on simultaneously, in which case the metal particles are sensitized to some extent before they are deposited and while they are yet in the vapor phase; or in any desired sequency or combination of steps, with satisfactory results.

In applying alkali metal to. the surface of the cathode of a phototube we have found it advantageous to attach to the tube 9, as shown in Fig. 6, a tube 24 constricted as at 25, leading to a high vacuum pump. Movably inserted in the tube 24 so as to engage the constriction is a gun-shaped iron capsule 26 loaded with suitable alkali metal. This capsule consists of an iron cylinder 21 movable by magnetic means, equipped on one end with a projecting nozzle 28 andclosed on the opposite end by a threaded cap 29. Two mica washers 30 of a diameter equal to that of the cylinder 21 and having starlike or other suitable projections are clamped to the cylinder, one at each end of the cylinder, and serve thermally to insulate the capsule from the tube 24 and at the same time permit the passage of gases. Withdrawing the capsule from the constriction temporarily, the system is evacuated. Coil 3| is then placed about the tube 24 near the center of the capsule and connected to a high frequency induction furnace and excited for a time sufficient to melt the alkali metal. This causes an atomic beam of such metal of conical shape to emerge from the nozzle and a quantity of such metal to be deposited on the inner surface of the tube 24 about the'entrance to the tube 9, whence it may be driven, by the application of a flame, to be deposited uniformly on the walls of the tube 9. Successive layers of the alkali metal may be similarly deposited. By suitably extending the nozzle 28 and tapering or otherwise suitably shaping the opening thereof, the alkali metal may be deposited directly on the walls of the tube 9.

Instead of forming a photosensitive surface by depositing alkali metal upon a given base or treating the metal either during or after deposition with activated and dissociated hydrogen, we have obtained good results by bubbling molecular hydrogen through molten alkali metal and suddenly expanding the I resulting vapor into an evacuated tube to form a deposit upon the cathode base, which is kept cool for this purpose. An arrangement of apparatus for carrying out this method is shown in Fig. 8 in which container 31 holds a supply of molten alkali metal 38, preferably heated to a temperature of about 350 C. by electric induction coil 39 or other suitable means, through which molecular hydrogen from container l is bubbled, the valve 5 being closed, to form vapor which fills the space in container 31 above the surface of the molten metal and the outlet 40. When the pressure'of the vapor thus formed becomes substantially equal to that of the molecular hydro gen, thevalve 5 is suddenly opened, the valve It) being preferably closed, to allow the vapor to expand, and when expanded, to be deposited on the surface of the cathode I2.

We have found it advantageous in some cases to provide a shield, which may be in the form of a cylindrical grid or perforated plate or other suitable device, glazed or coated with a material suitable for preventing recombination of atomic hydrogen, around or near one or more of the filaments, as indicated at 44 in Fig. 3.

High vacuum phototubes, especially those used in television for the transmission of images, deteriorate with use, among other reasons because of the accumulation of gases evolving from the electrodes and the envelope. We have found it possible to overcome this difficulty to a great extent by so constructing a tube of this type as to provide independent sources of hydrogen and electrons within the tube for resensitizing and stabilizing the photosensitive surface, and means for constantly counteracting the accumulation of such gases, and thus to maintain the requisite high vacuum. Ir our improved type of tube for such use, as shown in Fig. '7, we provide a suitable number of filaments, in thisinstance four, indicated at 33, 34, 35 and 36. Filament 33 is made of tantalum or other suitable material which has been treated in an H2 atmosphere so as to absorb a considerable quantity of such gas. After the tube has been sealed, the filament 33 is heated by the application of a suitable current until the requisite amount of H2 is released. Filament 34, which is made of tungsten, is then heated. The heat given oiI by filament 34 activates and substantially dissociates the H2 released by filament 33 to sensitize the alkali metal surface 56. The filament 34 also activates and substantially dissociates the residual gases, including gases evolved from the envelope and metal parts within the tube. Filament 35, which is made of a relative thick wire of an alloy of barium and tantalum known as batalum or other getter material, is then heated and absorbs or otherwise disposes of such residual gases. Next, filament 36, which is made of platinum or other heat resisting metal coated with alkaline earth oxides or other thermionically emissive material, discharges electrons bombarding the photosensitive surface I3 of the tube, further sensitizing and stabilizing it. When the .desired sensitivity and stability have been attained, the current is cut oif from the filament 36 and the filaments 34 and 35 are again heated simultaneously so as to dispose of residual gases remaining in the tube. It is not essential that the precise order of steps thus outlined be followed, and one or more of them may be repeated or two or more of them carried on simultaneously or intermittently, but preferably the concluding step should be an effective removal of the residual gases. If desired, potassium or other suitable alkali metal may be included for restoring the photosensitive surface.

In the finished tube, the circuits of the filaiment's 33 and 36 are open or suitably shorted; the filaments 34 and 35 may or may not be connected to receive current during the use of the tube and thus serve to maintain the high vacuum. When desired, the filaments 33 and 36 may be connected to be heated as above indicated for the purpose of restoring the sensitivity of the tube.

Instead of providing four filaments, each of difierent material, three filaments consisting of heating spirals of one or more materials such as tungsten or platinum, may be employed, each filament being arranged to heat a capsule pro- .vided with suitable openings and containing respectively tantalum powder, getter material, and electron emissive material. In such case, the necessary heat for activating and dissociating the gases-is provided by a fourth filament, or a common filament may be used for releasing and an additional filament coated with for activating and dissociating hydrogen. If desired, supplemental heating means may be added.

In the construction of a tube of this type, the filaments and other parts having been suitably placed, instead of fully evacuating the tube, the tube may be evacuated to about 0.001 mm. Hg pressure and the processes involved in the operation of the tube relied upon for further evacuating it.

The application of our method to the type of phototube employed in television, known as the iconoscope, makes it possible to use potassium for the photosensitive surface, which is a mosaic structure as indicated at 5'6 in Fig. '7, with highly satisfactory results not hitherto obtainable because of inability to attain sufiicient sensitivity for proper spectral distribution. In the construction of an iconoscope the electron gun usually provided for scanning may be used for sensitizing and stabilizing according to our method, and if desired, at a speed relatively lower than the usual scanning speed.

Having thus disclosed our invention, we claim:

1. The method of forming a photosensitive surface consisting in bubbling molecular hydrogen through molten alkali metal, expanding the resulting vapor and applying the expanded vapor to a suitable surface.

2. The method of forming a photosensitive surface consisting of depositing alkali metal upon a suitable surface and bombarding the metal with atomic hydrogen as it is deposited.

3. The method of forming a photosensitive surface consisting of depositing alkali metal upon a suitable surface and bombarding the metal with atomic hydrogen and thermionically during deposition.

4. The method of forming a photosensitive surface consisting of applying alkali metal to a surface of suitable material by vaporizing the metal, exposing the vapor to atomic hydrogen, causing the vapor to be deposited upon such surface, and electronizing the resulting surface.

5. The method of rendering an electrode in a phototube photosensitive consisting of subjecting hydrogen of suitable humidity to a potential of approximately 750 volts in a closed chamber until a substantial predominance of Balmer lines appear in the spectroscope, releasing the resulting gas into the said tube, and depositing alkali metal upon the surface of said electrode in the presence of said gas.

6. The method of rendering an electrode in a phototube photosensitive consisting of subjecting hydrogen of suitable humidity to a potential of approximately 750 volts in a closed chamber for about five seconds, releasing the resulting gas into said tube, and depositing alkali metal upon the surface of said electrode in the presence of said gas.

7. The method of forming a photosensitive surface consisting of bubbling molecular hydrogen through molten alkali metal, expanding the resulting vapor, applying the expanded vapor to a suitable surface, and bombarding the resulting surface with electrons.

8. A cathode for a phototube comprising a deposit of alkali metal upon a surface of suitable material, said metal having adsorbed thereon a layer of gas consisting largely of atomic hydrogen.

9. A cathode for a phototube comprising a deposit of alkali metal upon a surface of suitable material, said metal having adsorbed thereon an electronized layer of gas consisting largely of atomic hydrogen.

10. The method or forming a photosensitive surface consisting of depositing alkali metal upon a suitable surface, and bombarding said alkali metal with a gas consisting largely of atomic hydrogen.

11. The method of forming a photosensitive surface consisting of depositing alkali metal upon a. suitable surface, bombarding said alkali metal with a gas consisting largely of atomic hydrogen and bombarding the resulting surface thermionically.

JOSEPH 'IYKOCINSKI TYKOCINER.

JAKOB KUNZ.

\ LLOYD PRESTON GARNER. 

